The present invention relates to automatic meter reading (AMR) and, more specifically, to a meter interface unit for use in a one way automatic meter reading system. In such a “one-way” system, as opposed to an “on-demand” system, the meter interface unit transmits only. Therefore the cost of the unit is kept to a minimum by reducing the necessary radio circuitry by the elimination of the receiver required by on-demand systems.
There are two types of automatic meter reading system architecture, “mobile” and “fixed” network systems. In the mobile type the meters are read by hand-held or mobile readers which travel through the vicinity of the meters to be read. In this case, a radio transmitter which has only a relatively short range is needed. In the second fixed network type of automatic meter reading system, a fixed network is provided to which each meter interface unit transmits. In this case, the transmitter in the meter interface unit requires a relatively long range if the number of receiving stations in the fixed network is to be kept the reasonable number.
1. Technical Problem
In many circumstances where a fixed network is to be installed there is a technical problem in finding suitable sites for the first tier of receiving stations.
2. Prior Art Solutions
This can be overcome by using hand-held or mobile readers for this first tier. GB-A-2330279 (WO 99/18700) describes a way of generating and receiving a short low-energy transmission that can be made very frequently, allowing the messages to be received with a minimum of delay. If the transmissions can be made sufficiently often and the radio range is adequate, a vehicle can travel at a normal road speed of, say, 40 km/h and still pick up all the readings.
Disadvantages of the Prior Art
Unfortunately because of the restriction on the battery power available and in order to minimise channel congestion these “blind” transmissions must be very short and at high data rates. This implies a wide radio band width and consequential low sensitivity for the reader's receiver hence giving a restricted range. If the same fast frequent transmissions are used in a fixed network system the range will be very limited,.requiring a very high density of receiving points.
Installation of the meter interface units is one of the set up costs of installing an automatic meter reading system. Clearly it is not practical to replace the meter interface unit after installation if a local site for a first tier receiving station is found.
Example of One Way Fixed Network Requirements
In an average low power mobile AMR system a range of about 100 m can be achieved between the MIU and the reader. This corresponds to a scheme with a link budget of about 110 dB (10 dBm transmitter and a −100 dBm sensitivity receiver) and an operating frequency of 400–1000 MHz. If the reading vehicle is travelling at about 40 km/h the maximum time between the transmissions is limited to about 5 seconds. This ensures that the reader has the opportunity to receive at least two transmissions from each MIU. The duration of the data message will be in the range of 1–2 mS and if the message is about 120 bits long a data rate of about 100 kbps is needed. If the modulation used is Direct Frequency Shift Keying (DFSK) the receiver noise bandwidth must be approximately 400 kHz.
In a purely fixed network system the MIU is always within range of the reader and so the rate of transmissions is less important. A few times per day might be adequate with a typical rate of once every 4 hours. This means that the transmissions could last much longer with the same battery capacity and so the message could be sent at a slow data rate of say 200 bits/second. If the same data packet and modulation index is used the receiver noise bandwidth could be reduced to about 4 kHz. This could allow an improvement in the receiver sensitivity of about 20 dB. Elevating the receiver antenna and providing a modest amount of antenna gain could give a further 10 dB of link budget.
The total gain over the fast data rate mobile system is therefore 30 dB or 1000× power gain. In theory with free space propagation this would give a √1000 or 333 fold increase in the 100 m range. In practice with average urban conditions the improvement will be 7 to 20 times. This would mean that a 100 meter range wide band mobile system would achieve about 700 meters to a narrow band fixed receiver.